This invention relates to DC link inverter arrangements containing, for example semiconductor switches which are turned on and off
To maximize the volt-second product applied to a motor load in a pulsed DC power inverter motor drive, a square pulse should be provided. A square-pulse provides the maximum volt-second area producing the highest amount of work done per ampere of supplied current or of the quantity of flux produced in the motor. When semiconductor switches are turned on and off rapidly in an inverter motor drive, square pulses are produced which place large voltage stresses on the switches. When the switches are turned off they must undergo a transition from carrying the full DC load current to blocking the full DC source voltage. Because most high-power devices such as thyristors, bipolar junction transistors, insulated gate bipolar transistors and the like have significant stored charge which must be removed before the device can be turned off completely, the forward current does not drop to zero immediately. Consequently, both a voltage across the device and a current through the device are present during the transition, which leads to power dissipation in the device. This power dissipation produces heat and places a thermal limit on the frequency at which the inverter may operate in addition to reducing the overall efficiency of the inverter. Furthermore, the steep edges of the voltage pulses can lead to significant electromagnetic interference with other equipment.
In so called soft-switching inverters, switch transitions occur under zero voltage or zero current conditions so that little or no power is dissipated. The opportunity to switch at zero voltage or zero current is typically provided by the addition of one or more inductor-capacitor (LC) tanks to the circuit. The resonant dynamics of these additional components can be controlled to insure that the trajectories of the voltages or currents go to zero and the sinusoidal current output of such circuits can be controlled by using either pulse width modulation or pulse density modulation techniques, but additional circuitry is required for the use of pulse width modulation because of the fixed width of the resonant pulse.
A basic resonant link inverter suffers from two fundamental problems. It produces a peak voltage which is two or more times the DC supply voltage, thereby requiring switches with voltage ratings of twice the DC supply voltage and it provides a reduced bus voltage volt-second product area. The first difficulty is particularly important when using power MOSFETs for the controlled switch because the channel resistance of these devices varies approximately with V.sub.rated.sup.2.5 where V.sub.rated is the rated forward blocking voltage of the device. This increased resistance leads to greater conduction losses in the switching device. For minority carrier devices, the dependence of the saturation voltage on the forward voltage blocking rating is not as great, typically varying only linearly rather than exponentially with V.sub.rated. Nevertheless, some higher level of undesirable conduction losses must be accepted in order to accommodate the high peak voltage.
The problem of reduced volt-second area is also a matter of concern. Ideally, to transfer maximum energy for a fixed operating frequency and supply voltage V.sub.S, an inverter should supply full voltage for one switching period and then instantaneously switch to zero, thereby providing a rectangular voltage pulse having maximum volt-second area. For the same pulse period and peak voltage value, the volt-second area of the resonant voltage pulse from a tank circuit is lower than that of a rectangular pulse because of the inherent shape of the resonant voltage.
The Lee et. al. Pat. No. 4,992,919 discloses a parallel resonant converter for zero voltage switching including a tank circuit with a varactor diode providing a capacitance which is variable with increasing applied voltage. In this circuit, the variable capacitor is selected so that the output voltage of the converter can remain at its rated value of the zero voltage switching condition.
The Mekanik et. al. Pat. No. 5,760,495 discloses an inverter circuit for an uninterruptable power supply including a ferroresonant capacitor connected across the terminals for transformer circuitry for providing output which is linearly related to the input to the primary winding of the winding of the transformer.
A variable voltage capacitor is disclosed in the Yandrofski et al. Pat. No. 5,721,194 which describes various applications of capacitors incorporating ferroelectric films and the Yamamoto et al. Pat. No. 4,360,762 discloses a non-linear capacitor in a resonant circuit for the purpose of shaping a switching pulse in a starter switch for a fluorescent lamp.